there are a couple rotors that might work for a smaller package, also the standard boxster calipers are slightly smaller so would fit a smaller rotor better. AS for the bracket I think is doable I t would have to change a little so the caliper is not spaced out as far from the hub. going down to a 290mm rotor you would have to lose about .75" off the bracket. this only leaves about .5" of threads for the caliper bolts. prob enough but it would be a safer bet to switch to a stud that screws into the bracket and the use a nut to secure the caliper. pretty much the way stoptech does all their brackets.

while they have the same 130mm bolt centers, they are different sizes also the front calipers have 36/40mm pistons and the rears have 28/30mm pistons. What this means is the rear calipers are much too small to use on the front. Ebay can be good if you find a deal but those dont come around all the time. If you check out car-part.com you can find the 911 calipers for 150-200 each and the standard boxster calipers i found a bunch for 100 each.

Also, the choice of brake pads can make a difference... Some pads are crazy noisy, others only make noise at certain brake pressures/speeds. My first set of pads was noisy, my current ones are silent so far.

You need the front calipers off a 99-2005 911 Carrera 2/4 996, 2000-2008 Boxster S or a 2006-2008 Cayman, Porsche part #996.351.425 & 996.351.426, I got mine off a 2002 Boxter S (3.2L)

They may say Porsche on them or they may say Brembo, regardless they should have the part # on them as part of the mold (raised numbers) and small raised letters saying Brembo. Doing some research into pads Porsche owners don't seem to think there's much difference between the regular Boxter (non-Brembo) and Boxter S (Brembo) calipers so you might be able to use those for cheaper.